WHERE Clause with CASE WHEN....THEN....ELSE ... END with columns = > and IS NULL conditions

Try the clean method:

IF IsNULL(@intSourceID,0) > 0
Begin
        SELECT     PriceRevID, PriceID, SourceID
        FROM         dbo.PriceListRev
        WHERE     SourceID = @intSourceID
End
Else 
Begin
        SELECT     PriceRevID, PriceID, SourceID
        FROM         dbo.PriceListRev
End